A common finding in clinical audiology is that hearing-impaired listeners with similar audiograms perform differently on measures of speech perception and differ in the benefits they receive from amplification. One reason for the lack of homogeneity among hearing-impaired listeners is that the audiogram provides only a rough estimate of cochlear pathology. It is possible that hearing-impaired listeners differ in their underlying cochlear pathology and pure-tone thresholds lack the sensitivity to distinguish individuals. Cochlear pathology alters cochlear transduction processes. Mechano-electric transduction processes are modified, either directly through changes in hair cell structure, or indirectly through structures on which hair cell transduction relies. An objective measure that characterizes these processes will describe the physiologic consequences of cochlear disorders and supply a more accurate description of sensorineural hearing loss then presently available. The long term goal of this research is to develop clinical measures of cochlear transduction to provide a new categorization scheme of sensorineural hearing loss based on the underlying pathophysiology. With future developments in hair cell regeneration and gene therapy, this information will be useful for targeting therapeutic agents towards physiologic sites as well as for developing new signal processing algorithms for hearing aids and cochlear implants. Indices describing cochlear transduction processes are estimated from an engineering-based nonlinear systems identification procedure applied to cochlear potentials and from a modeling approach applied to distortion product otoacoustic emissions. One specific aim of this research is to determine the optimal signal parameters to obtain indices of cochlear transduction in humans. In an animal model, the aims are to determine the influence of the middle ear transfer function on cochlear transduction processes, and to determine the sensitivity and specificity of cochlear transduction indices to distinguish cochlear pathology in ears with permanent hearing loss.